This invention relates generally to permanent magnet machines, and more particularly, to the manufacture of surface permanent magnet rotors for a permanent magnet machine.
It is to be understood that the present invention relates to generators as well as to motors, however, to simplify the description that follows, a motor will be described with the understanding that the invention also relates to generators. With this understanding, rotors including permanent magnets previously utilized curved magnets adhesively fixed to the periphery of the rotor or permanent magnets embedded in a steel rotor core.
Adhesive bond lines between laminations and between permanent magnets and the rotor cores are subject to failure during operation. Moreover, motor or generator heating and environmental conditions degrade the integrity of the adhesive, potentially leading to eventual magnet separation and rotor failure.
To help retain surface or adhesive mounted permanent magnets, a thin retaining cylinder, usually of metallic or wound fibrous construction, is employed. The use of such a thin retaining cylinder does have detrimental effects on machine performance and efficiency. In addition, the required cylinder thickness for a larger high speed motor or generator can make the use of such a can for these applications impractical.
Permanent magnets embedded in the steel core are retained by thin webs stamped into the lamination material. Determining the width of the thin webs involves a trade-off concerning flux leakage versus structural integrity. Making the web wider allows for higher speed operation and greater rotor robustness at the expense of greater flux leakage, more magnet cost, and lower machine output and efficiency.
There is thus a need to develop a permanent magnet machine that is not subject to magnet separation and adhesive or web failure, but rather has high mechanical integrity, and preferably that may be produced at a lower cost than that of current permanent magnet motors.
The present invention provides a composite powder metal disk for a rotor assembly in a permanent magnet machine, the disk having an inner annular magnetically conducting segment of soft ferromagnetic powder metal compacted and sintered to a high density. The disk further comprises an outer annular permanent magnet segment of alternating polarity permanent magnets. The permanent magnet segment may be a continuous magnet ring with regions of alternating polarity around the circumference of the disk or may be discrete permanent magnets separated from each other by spaces or by non-ferromagnetic powder metal segments pressed and sintered to a high density. In a further embodiment, a rotor assembly is provided having a plurality of the composite powder metal disks axially stacked along and mounted to a shaft. There is further provided a method of making such a composite powder metal disk and rotor assembly in which a die is filled according to the desired surface permanent magnet pattern, followed by pressing the powder metal and sintering the compacted powder to achieve a high density composite powder metal disk of high structural stability. The permanent magnets may be hard ferromagnetic powder metal pressed and sintered with the soft ferromagnetic and optional non-ferromagnetic powder metal, or may be prefabricated magnets adhesively affixed in the disks after sintering the powder metal portions. These disks are then stacked axially along a shaft with their magnetic patterns aligned to form the powder metal rotor assembly. A permanent magnet machine incorporating the powder metal rotor assembly of the present invention is simpler to manufacture and at a lower cost than prior surface permanent magnet rotors.